Method and apparatus for forming an extrusion

ABSTRACT

A method of forming an extrusion comprises passing a first, plastics material ( 12 ) through a first channel ( 104 ) in a die, passing a second, plastics material ( 14 ) through a second channel ( 112 ) in the die, and passing a third, plastics material ( 16 ) through a third channel ( 314 ) in the die. At least one of the materials is passed through a metering gap into contact with the other other materials to cause the at least one material to bond with the other materials as the materials pass through the die.

[0001] The present invention relates to a method and apparatus forforming an extrusion, and particularly but not exclusively to forming asealing element or strip.

[0002] U.S. Pat. No. 4,328,273 discloses a sealing element forattachment to a door or window, the sealing element having a resilientfoam core enveloped by a wrap-around outer cover having a flexibleportion and a relatively rigid portion for attaching the seal to anotherstructure. However, a disadvantage of such sealing elements is that thecover and the core are produced separately and combined at a later stagein production.

[0003] The present invention seeks to provide an improved method of andapparatus for forming an extrusion.

[0004] Accordingly, the present invention provides method of forming anextrusion comprising: passing a first, plastics material through a firstchannel in a die; passing a second, plastics material through a secondchannel in said die; and passing a third, plastics material through athird channel in said die; and passing said second material through ametering gap means into contact with said first and third materials tocause said second material to bond with said first and third materialsas said first and third materials pass through the die.

[0005] Advantageously, the second material contacts said first and thirdmaterials at different locations in the flowpath of said secondmaterial.

[0006] In a preferred form of the invention each of the three plasticsmaterials is introduced into the die in a heat softened flowable state.The flowable state may be a liquid, semi-liquid or viscous state.

[0007] Preferably, the second material is applied as a coating to atleast a portion of said first and third materials as said first andthird materials pass through the die.

[0008] Advantageously, the second material retains said first and thirdmaterials in spaced relationship in the extrusion.

[0009] Preferably, the third material forms a rigid or partially rigidstrip in said extrusion.

[0010] In a preferred form of the invention the third material forms arigid or partially rigid strip bonded to said foam substrate.

[0011] The present invention also provides a die for forming an extrudedprofile, the die comprising: a first resin channel means for receiving afirst plastics material in a flowable state; a second resin channelmeans for receiving a second plastics material in a flowable state; anda third resin channel means for receiving a third plastics material in aflowable state; wherein said die further comprises a metering gap meansfor directing said second material into contact with said first andthird materials to cause said second material to bond with said firstand third materials as said first and third materials pass through thedie.

[0012] In a preferred form of the invention the second material isdirected into contact with said first and third materials at differentlocations in the flow path of said second material.

[0013] Advantageously, a first metering gap is provided for directingsaid second material into contact with said first material and a secondmetering gap is provided for directing said second material into contactwith said third material.

[0014] Preferably, the first and third channel means have respectiveextrusion orifices wherein said first channel means extrusion orifice iswithin said second channel means intermediate the ends thereof and saidthird channel means extrusion orifice is at or adjacent the downstreamend of said second channel means. The metering gap is formed betweeneach extrusion orifice and the inner wall of said second channel means.

[0015] The present invention also provides an apparatus for forming anextrusion comprising means for directing first, second and third heatsoftened flowable plastics material continuously into a die wherein thedie comprises: a first resin channel means for receiving said firstplastics material in a flowable state; a second resin channel means forreceiving said second plastics material in a flowable state; and a thirdresin channel means for receiving said third plastics material in aflowable state; and a metering gap means for directing said secondmaterial into contact with said first and third materials to cause saidsecond material to bond with said first and third materials as saidfirst and third materials pass through the die.

[0016] Accordingly, the present invention provides a method of formingan extrusion comprising: introducing a first, foam material substrateinto a die opening in a die and extruding said material through saiddie; introducing a second, plastics material into said die in a liquidstate and applying said second material as a coating to said substrateas said substrate passes through the die; and introducing a third,plastics material into said die in a liquid state to bond with saidfirst material substrate as said first material substrate passes throughthe die opening.

[0017] In a preferred form of the invention the third material forms arigid or partially rigid strip bonded to said foam substrate.

[0018] The present invention also provides a die for forming an extrudedprofile comprising a coated substrate, the die comprising: a die openingthrough said die for passing the substrate material therethrough; afirst resin channel for receiving a first material in a liquid state forforming the coating on said substrate material; and a second resinchannel for receiving a second resin material in a liquid state; whereinsaid first resin channel has a metering gap for metering said firstliquid resin to bring said first liquid resin into contact with saidsubstrate material so as to form said coating thereon; and said secondresin channel opens into said die opening to bring said second liquidresin into contact with said substrate material for forming a lateralextension to said substrate.

[0019] The present invention is further described hereinafter by way ofexample, with reference to the accompanying drawings, in which:

[0020]FIG. 1 is an end elevation of a back plate of a preferred form ofdie according to the present invention;

[0021]FIG. 2 is a partial section along the line 2-2 of FIG. 1;

[0022]FIG. 3 is an opposite end elevation of the back plate;

[0023]FIG. 4 is a view similar to that of FIG. 1 of a middle plate ofthe preferred form of die according to the present invention;

[0024]FIG. 5 is a view similar to that of FIG. 2 of the middle plate ofFIG. 4;

[0025]FIG. 6 is a view similar to that of FIG. 3 of the middle plate ofFIG. 5;

[0026]FIG. 7 is a view similar to that of FIG. 1 of a forward plate ofthe preferred die according to the present invention;

[0027]FIG. 8 is a view similar to that of FIG. 2 of the plate of FIG. 7;

[0028]FIG. 9 is a view similar to that of FIG. 3 of the plate of FIG. 7;

[0029]FIG. 10 is a view similar to that of FIG. 1 of a front plate ofthe preferred form of die according the present invention;

[0030]FIG. 11 is a view similar to that of FIG. 2 of the front plate ofFIG. 10;

[0031]FIG. 12 is a view similar to that of FIG. 3 of the front plate ofFIG. 10;

[0032]FIG. 13 is a partial section through the assembled die of FIGS. 1to 12;

[0033]FIG. 14 is a diagrammatic representation of an extrusion apparatususing the die of FIGS. 1 to 13;

[0034]FIG. 15 is a section through an extrusion produced by the die ofFIGS. 1 to 13;

[0035]FIG. 16 diagrammatically represents the flow paths of materialsthrough the assembled die of FIGS. 1 to 13;

[0036]FIG. 17 is an end elevation of a back plate of second a preferredform of die according to the present invention;

[0037]FIG. 18 is a partial section along the line 18-18 of FIG. 17;

[0038]FIG. 19 is an opposite end elevation of the back plate;

[0039]FIG. 20 is a view similar to that of FIG. 17 of a front plate ofthe second preferred form of die according the present invention;

[0040]FIG. 21 is a view similar to that of FIG. 18 of the front plate ofFIG. 20;

[0041]FIG. 22 is a view similar to that of FIG. 19 of the front plate ofFIG. 20;

[0042]FIG. 23 is a partial section through the assembled die of FIGS. 17to 22;

[0043]FIG. 24 is a diagrammatic representation of an extrusion apparatususing the die of FIGS. 17 to 23;

[0044]FIG. 25 is a section through an extrusion produced by the die ofFIGS. 17 to 23; and

[0045]FIG. 26 is a view similar to that of FIG. 16 showing the flowpaths of materials through the assembled die of FIGS. 17 to 24.

[0046] Referring firstly to FIG. 15 this is a cross-section of a weatherseal 10 formed by extrusion using the die of FIGS. 1 to 13. Theextrusion has a core 12 formed of a first, resilient, foam material suchas a thermoplastic elastomer which is covered with a flexible orpartially flexible coating 14 such as a thermoplastic elastomer. Bondedwith the foam core 12 is a strip 16. Whilst the strip 16 is preferablyrigid, it can also be flexible or partially flexible. Finally, a rib orstrip 18 of flexible or partly flexible material is bonded to the strip16. The material forming the rib 18 is conveniently the same as thematerial forming the coating 14.

[0047] As can be seen from FIG. 15, the body of the extrusion has agenerally V-shape formed by two arms or limbs 14 a, 14 b. The rigid orpartly rigid strip 16 is generally L-shaped, having two arms or limbs 16a, 16 b with one arm 16 a bonded to the free end region of the arm 14 bof the body 14. The strip 18 is bonded to the edge region of the freearm 16 b of the L-shaped strip 16.

[0048] Referring to FIGS. 1 to 13 these show a preferred form of die 20according to the present invention which is formed by four plates, aback plate 100, a middle plate 200, a forward plate 300 and a frontplate 400.

[0049] FIGS. 1 to 3 show the back plate 100 which has an outer face 106and an inner face 108. A die opening in the form of a resin channel 104extends through the centre of the back plate and as can be seen fromFIGS. 1 and 3 is generally V-shaped, corresponding to the V-shaped bodyof the extrusion 10.

[0050] FIGS. 4 to 6 show the middle plate 200 which has a back face 206and a forward face 208. The back face 206 is intended to abut the innerface 108 of the back plate 100.

[0051] The middle plate 200 has a resin channel 204 which is intended asan extension of the resin channel 104 of the back plate 100.

[0052] The back plate 100 has two elongate, parallel channels 110 formedin the inner face 108. These channels open into the face 108 and areformed one above and one below the resin channel 104. The channels 110communicate through respective arms of a Y-shaped channel 112 to aninlet half 114, both of which are formed in the inner surface 108 of theback plate.

[0053] The middle plate 200 has channels 210 corresponding to thechannels 110 of the back plate but the channels 210 of the middle plateextend fully through the middle plate. The middle plate also has aninlet half 214 which, in the assembled die, combines with inlet half 114of the back plate to form an inlet for a second, coating material. Whenthe two plates are joined together with back face 206 of the middleplate abutting the inner face of the back plate, the channels 110, 210align such that material which is fed under pressure into the inlet 114flows through the channels 112, 110 into the channels 210.

[0054] The channels 210 of the middle plate open into a generallyrectangular recess 216 formed in the forward face 208 of the plate. Whenthe middle plate is joined to the forward plate 300 the recess 216 formsa reservoir for the material.

[0055] As can be seen in FIGS. 5 and 6 the recess 216 carries anupstanding tubular portion or extension 218 which forms an extension ofthe resin channel 204. As can be seen from FIG. 6 extension 218 has across-sectional shape which corresponds to that of the desiredcross-sectional shape of the body 14 of the extrusion 10. The extension218 projects above the forward face 208 of the middle plate 200 andengages through the forward plate 300 as described below.

[0056] FIGS. 7 to 9 show the forward plate which has a middle face 306and a front face 308. The middle face 306 abuts the forward face 208 ofthe middle plate in the assembled die and a through-hole or channel 310which extends fully through the forward plate 300 aligns with theright-hand end of the lower channel 210 of the middle plate as seen inFIG. 6. This allows the second material to flow from the reservoir 216and channel 210 into the through-channel 310.

[0057] The forward plate 300 also has a resin channel 304 which is anextension of the resin channels 104, 204. Channel 304 has across-sectional shape corresponding to that of the projection 218 but isdimensioned so as to provide a preferably uniform gap between the innerwall 304 a of the channel 304 and the outer surface 218 a of theextension 218. This provides a metering gap for the second material andallows the second material to flow from the reservoir 216 through thegap between the channel 310 and the extension 218. The material flowspast the open end of the extension 218 to contact the foam extruding outof the extension 218 and bond with the foam outer surface.

[0058] The front face 308 of the forward plate 300 also has a channel312 and an inlet half 314 which are open to the front face 308. Thefront plate 400 has a cooperating inlet half 414 and channel 412 formedin the inner face 406 such that when the die is assembled with the innerface 406 abutting the forward face 306 the inlet halves 314, 414 andchannels 312, 412 combine to form a further inlet and channel for thethird material. The channel 312 has an end portion 320 which, in theassembled die, aligns with a through-channel 420 in the front plate 400.The channel 420 has a relatively large opening in the inner face 406 ofthe front plate 400 but the channel narrows to its opening 422 towardsthe outer face 408 of the plate 400. This brings a portion of the thirdmaterial through to form the strip 18 and the opening 422 is shaped toprovide the shaped flexible strip 18 of the extruded product.

[0059] The continuation of the resin channel 104, 204, 304 is shown at404 in FIGS. 10 and 12. The length of the extension 218 of the middleplate 200 is such that it projects into the channel 404 but does notextend to the outer face 408 of the plate 400. As can be seen in FIGS.10 and 12 the opening of the channel 404 in the outer face 408 is ofreduced dimension compared to the opening in the forward face 406.

[0060] The front plate 400 is also provided with a furtherthrough-channel 430 which is of generally L-shape and as can be seenfrom FIG. 10 its opening into the forward face 406 lies adjacent one armof the extrusion channel 404. However, as can be seen from FIG. 12 thetwo channels 404, 430 join at a point partway through the front dieplate 400 such that the openings of the channels 404, 430 and 422 in theouter face 408 are combined to the profile of the end product shown inFIG. 15.

[0061] The front face 308 of the forward plate 300 has an L-shapedrecess 330 which aligns with the recess 430 of the front plate 400 whenthe die is assembled but is wider than the recess 430 to form arelatively small reservoir. Resin is supplied to the recess 330 andchannel 430 by way of the channel 312, one limb of the channel 430bridging the gap between the end 320 of the channel 312 and the recess330 to allow the passage of resin.

[0062]FIG. 14 shows an assembled system for producing the extrusion, thesystem having a first source 50 of the foam material, a second source 52of the second, plastics material which is rigid or partially rigid whenset or cured, and a third source 54 of the third, plastics materialwhich is flexible or partially flexible when set or cured. Thesematerials are extruded through the die 20 to form the extruded product.

[0063] In operation, the die 20 is firstly heated to a set temperatureand the foam material is fed in a heat softened flowable state throughthe die channel 104, 204, 304, 404 of the die. Once the foam extrusionbegins to exit the die through the combined openings 404, 422, 430 thesecond material is fed in a heat softened flowable state or liquid formfrom the source 54 through the inlet 144, 214, the channels 112, 212,the channels 110, 210, the reservoir 216, between the walls of thechannel 304, 404 and the protrusion 218 until it comes into contact withthe outer surface of the extruding foam. The two materials naturallybond together and exit the die. At the same time, the third material isfed again in a heat softened flowable state or liquid form into the diethrough the inlet 314, 414 and channels 312, 430 and 330 such that itflows into contact with and bonds naturally with the foam material and,where applicable, the second material at or adjacent the outer end ofthe extension 218.

[0064] Finally the second material is also fed from the channel 210 andreservoir 216 through the channel 310 into the channel 420 where itcontacts and naturally bonds to the third material as it flows throughthe channel 430 in the front plate 400.

[0065]FIG. 16 shows in diagrammatic form the flow of the materials 51through the die.

[0066] The foam material 12 is passed through the die channel 104, 204,304, 404 of the die and the second, flexible material 55 which forms thecoating or skin 14 and the rib 18 is fed into the die through the inlet114, 214. The third, rigid or semi-rigid material 53 is fed into the diethrough inlet 314 to form the strip 16 which is bonded to both thesubstrate 51 and the material 55 which forms the coating 14 and flexiblestrip 18.

[0067] It will be appreciated, therefore, that in the above process allthree materials bond together through a combination of temperature andpressure within the die with the final profile of the extruded productbeing shaped by the openings 404, 422,430 in the front face 408 of thedie plate 400.

[0068] Referring now to FIGS. 17 to 26 these show a second preferredembodiment of die and extrusion process according to the presentinvention.

[0069] Referring firstly to FIG. 25, this is a cross-section of aweather seal 10 formed by extrusion using the die of FIGS. 17 to 23. Theextrusion has a first portion or core 12 formed of a first, plasticsmaterial which in this example is a resilient, foam material such as athermoplastic elastomer.

[0070] The extrusion has a second portion 14 formed by a second plasticsmaterial such as a thermoplastic elastomer. This forms a flexible orpartially flexible coating or body which encloses the core 12.

[0071] As can be seen from FIG. 25, the body 14 of the extrusion has agenerally V-shape formed by two arms or limbs 14 a, 14 b with the core12 enclosed within the base 15 of the V. A third portion of theextrusion is formed by a third plastics material which forms a generallyplanar strip 16 which is fully enclosed by the material of the limb 16a. The latter effectively forms a coating over the strip 16. Thematerial of the body 14 thus encloses and maintains the core 12 andstrip 16 in spaced relationship. Whilst the strip 16 is preferablyrigid, it can also be flexible or partially flexible.

[0072] The body 14 also has a flange 18 which extends laterally of thelimb 14 b. The flange is generally U-shaped having two limbs 18 a, 18 bwith the open end of the U being closed by the limb 16 b. Thus theflange is formed with an internal cavity or air space 19 which extendslongitudinally of the extrusion. The flange base is shaped like anarrowhead in cross section as a retension feature in use.

[0073] Referring to FIGS. 17 to 23 these show a preferred form of die 20according to the present invention which is formed by two plates, a backplate 100 and a front plate 400. Two plates are used for conveniencehere and it will be appreciated that more than two could be used.

[0074] FIGS. 17 to 19 show the back plate 100 which has an outer face106 forming the rear face of the die, and an inner face 108. Fourgenerally parallel resin channels 104 extend through a central region ofthe back plate and open into a generally rectangular recess 116 formedin the inner face 108 of the plate. When the back plate is joined to thefront plate 200 the recess 116 forms a reservoir for the second plasticsmaterial.

[0075] As can be seen in FIGS. 21 and 22 the recess 116 carries in orabout its central region an upstanding portion or extension 118 which isformed by a number of tubular extensions 118 a, 118 b and 118 c.Extension 118 a is provided for the material which forms the core 12 ofthe extrusion and extension 118 b is provided for the material whichforms the strip 16. These tubular extensions have cross-sectional shapeswhich correspond to the desired shapes of the core 12 and strip 16 andconnect respectively to two elongate channels 110, 112 formed in theplate 100 through which the materials for the core 12 and the rigidstrip 16 are passed.

[0076] The tubular extension 118 c is provided for the flange 18 andbecause it is intended to form the cavity 19 it is provided with an airpassage in the form of a bleed hole 120 which allows air external to thedie to flow into the cavity 19 as it is formed. Without the bleed holethere would be a danger of the cavity being collapsed by externalpressure. The bleed hole 120 could, of course, be replaced by a furtherchannel (not shown) similar to channels 110, 112. This could be used topass one of the extrusion materials or a fourth plastics material tofill the cavity 19 with, for example, a foam material.

[0077] The tubular extensions are formed by walls 119 a, 119 b and 119 cwhich are generally of the same thickness and, as can be seen from FIG.18, extend beyond the inner face 108 of the back plate 100. The wall 119a generally defines the shape of the core 12 and at the outer end of theextension 118 a forms an extrusion orifice 120 a. The wall 119 bgenerally defines the shape of the strip 16 and at the outer end of theextension 118 b forms an extrusion orifice 120 b. The wall 119 cgenerally defines the shape of the cavity 19 and forms an orifice 120 c.As can be seen from FIG. 18, the walls 119 a and 119 c project by aboutthe same amount beyond the inner face of the back plate 100 whilst thewall 119 b projects substantially further.

[0078] The front plate 400 has an inner face 406 and an outer face 408forming the front face of the die. It is also provided with athrough-channel 430 which opens into the front face 408. At its outerend 432 (at the front face 408) the channel 430 is of a cross-sectionalshape which corresponds to that of the desired extrusion 10. The channel430 aligns with the recess 116 and the extension 118 of the back plate100 such that when the die is assembled the extension 118 projects intothe channel 430, as can be seen from FIG. 22. The difference in lengthsof the walls 119 a, 119 b and 119 c means that the walls 119 a and 119 cand thus the extrusion orifices 120 a, 120 b terminate within thechannel 430, approximately half way through the front plate 400 whilstthe wall 119b terminates at or adjacent the front face 408. Theextrusion orifices 120 a, 120 b could, of course, be provided at anysuitable location in the channel 430.

[0079] The channel 430 has an opening in the inner face 406 of the frontplate 400 which is larger than the outer end 432 but the channel narrowsor tapers inwardly towards its opening 432 in the outer face 408 of theplate 400. The walls 119 a, 119 b and 119 c of the extension and theinner wall of the channel 430 form a preferably uniform gap around mostof the periphery of the extension 118. This provides a metering gap forthe second material and allows the second material to flow from thereservoir 116 through the gap between the channel 430 and the extension118, and past the open ends of the extension 118 to contact and bondwith the foam and strip material extruding out of the extension 118.Although the metering gap between the walls 119 a, 119 b and 119 c ofthe extension and the inner wall of the channel 430, around most of theperiphery of the extension 118, is preferably uniform, it will beappreciated that the gap may vary to vary the thickness of the coatingof the second plastics material around the core 12 and strip 16.

[0080] The limb 14 a is formed by a correspondingly shaped portion 414of the channel 430. It will be appreciated that the external shape ofthe extrusion, including any flanges or limbs such as limb 14 a can bevaried by varying the shape of the outer end 432 of the channel 430.Thus, a number of different die front plates can be used with a varietyof back plates to give extrusions having a variety of cross-sectionalshapes with a variety of shapes of core 12, strip 16 and cavity 19.

[0081] An assembled system for producing the extrusion is shown in FIG.24. The system has a fist source 50 of the first resin, a second source52 of the second resin which is flexible or partially flexible when setor cured, and a third source 54 of the third resin which is rigid orpartially rigid when set or cured. These materials are extruded throughthe die 20 to form the extruded product.

[0082]FIG. 26 shows in diagrammatic form the flow of the materialsthrough the die.

[0083] The foam material 12 forming the core is passed through the diechannel 110, 118 a of the die. The flexible or partially flexiblematerial 14 which encloses the core is fed into the die through theinlet holes 104 into the reservoir 116 and passes around the extension118. The third, rigid or partially rigid material 16 is fed into the diethrough inlet 112 and then through tubular extension 118 b and orifice120 b to form the strip 16.

[0084] In operation, the three sources of resin are heated to providethe resins in liquid, semi-liquid or viscous form. The degree ofviscosity of each resin is not important provided the resins can flowthrough the die. The die 20 is also heated to a set temperature to allowthe resins to flow through the die. The core material is passed throughthe die channel 110 of the die and the strip material is passed throughthe die channel 112. Once the first and third materials begin to exitthe die through the combined openings 118 a, 118 b the second materialis fed in liquid form from the source 52 through the channels 104 andthe reservoir 116, between the walls of the channel 430 and theextension 118 until it comes into contact with the outer surfaces of theextruding first and third materials. The materials naturally bondtogether and exit the die as the complete extrusion.

1. A method of forming an extrusion comprising: passing a first,plastics material (12) through a first channel (104) in a die; passing asecond, plastics material (14) through a second channel (112) in saiddie; and passing a third, plastics material (16) through a third channel(314) in said die; wherein each said plastics material is introducedinto said die in a flowable state and at least one of said materials ispassed through a metering gap means into contact with at least one ofthe other of said materials to cause said at least one material to bondwith said at least one of said other of said materials as said materialspass through the die.
 2. A method as claimed in claim 1 wherein saidsecond material is passed through said metering gap means into contactwith said first and third materials to cause said second material tobond with said first and third materials as said first and thirdmaterials pass through the die.
 3. A method as claimed in claim 1 or 2wherein the second material contacts said first and third materials atdifferent locations in the flowpath of said second material.
 4. A methodas claimed in any of the preceding claims wherein each of the threeplastics materials is introduced into the die in a heat softenedflowable state.
 5. A method as claimed in claim 4 wherein said flowablestate is one of a liquid, semi-liquid or viscous state.
 6. A method asclaimed in any of the preceding claims wherein the second material isapplied as a coating to at least a portion of said first and thirdmaterials as said first and third materials pass through the die.
 7. Amethod as claimed in claim 6 wherein the second material retains saidfirst and third materials in spaced relationship in the extrusion.
 8. Amethod as claimed in any of the preceding claims wherein the thirdmaterial forms a rigid or partially rigid strip in said extrusion.
 9. Amethod as claimed in any of the preceding claims wherein the thirdmaterial forms a rigid or partially rigid strip bonded to said firstmaterial.
 10. A method as claimed in any of the preceding claims whereineach of said second and third materials is passed through a respectivemetering gap means into contact with at least one of the other of saidmaterials to cause each said second and third material to bond with saidat least one of said other of said materials as said materials passthrough the die.
 11. A die for forming an extruded profile, the diecomprising: a first channel through said die for passing a firstplastics material therethrough; a second channel for receiving a secondplastics material in a flowable state; a third channel for receiving athird plastics material in a flowable state; and a first metering gapmeans for metering said second plastics material to bring said secondplastics material into contact with at least one of said first and thirdplastics materials so as to bond therewith.
 12. A die as claimed inclaim 11 wherein said first channel extends into said second channel andterminates in a wall disposed between said first and second channels soas to form said metering gap means in said second channel.
 13. A die asclaimed in claim 11 or 12 wherein said third channel extends into saidsecond channel and terminates in a wall disposed between said second andthird channels so as to form said metering gap means in said secondchannel.
 14. A die as claimed in claim 11, 12 or 13 wherein said firstand third channels extend into and terminate in said second channel, andsaid first and third channels are arranged such that said first andthird materials are held in spaced relationship by said second materialin said extrusion.
 15. A die as claimed in any of claims 10 to 14wherein the second material is directed into contact with said first andthird materials at different locations in the flow path of said secondmaterial.
 16. A die as claimed in any of claims 10 to 15 wherein a firstmetering gap is provided for directing said second material into contactwith said first material and a second metering gap is provided fordirecting said second material into contact with said third material.17. A die as claimed in any of claims 10 to 15 wherein the first andthird channels have respective extrusion orifices wherein said firstchannel extrusion orifice is within said second channel intermediate theends thereof and said third channel extrusion orifice is at or adjacentthe downstream end of said second channel.
 18. A die as claimed in claim17 wherein the metering gap is formed between each extrusion orifice andthe inner wall of said second channel means.
 19. A die as claimed in anyof claims 11 to 18 wherein said first channel extends partially intosaid second channel and terminates in a wall disposed between said firstand second channels so as to form said metering gap means in said secondchannel, the arrangement being such that said second material at leastpartially surrounds said first material in said extrusion.
 20. A die asclaimed in claim 19 wherein said second material forms a coating on saidfirst material.
 21. A die as claimed in claim 11 wherein said thirdchannel extends into said second channel and terminates in a walldisposed between said second and third channels so as to form saidmetering gap means in said second channel.
 22. A die as claimed in claim19 wherein said first and third channels extend at least partially intoand terminate in said second channel, and said first and third channelsare arranged such that said first, second and third materials are incontact with one another in the extrusion.
 23. An apparatus for formingan extrusion comprising: means for directing first, second and thirdheat softened flowable plastics material continuously into a die; andwherein the die comprises: a first channel means for receiving saidfirst plastics material in a flowable state; a second resin channelmeans for receiving said second plastics material in a flowable state;and a third resin channel means for receiving said third plasticsmaterial in a flowable state; and a metering gap means for directingsaid second material into contact with said first and third materials tocause said second material to bond with said first and third materialsas said first and third materials pass through the die.